Process and apparatus for producing a pressurized product by low-temperature fractionation of air

ABSTRACT

For producing a pressurized product by low-temperature producing a pressurized product by low-temperature fractionation of air in a rectification system which has a high-pressure column ( 13 ) and a low-pressure column ( 14 ), a first feed airstream ( 12 ) is introduced into the high-pressure column ( 13 ), and an oxygen-rich fraction ( 38 ) from the low-pressure column ( 14 ) is brought ( 39 ) to pressure in the liquid state and introduced ( 41 ) into a mixing column ( 16 ). A second feed airstream ( 6, 15 ) is introduced into the lower region of the mixing column ( 16 ) and brought into countercurrent contact with the oxygen-rich fraction ( 41 ). The mixing column ( 16 ) is operated at a pressure (P M1S ) which is lower than the operating pressure (p HDS ) of the high-pressure column ( 13 ). A total airstream ( 1 ) which comprises the first and second feed airstreams is compressed ( 2 ) to a first pressure (p 1 ) which is lower than the operating pressure (p HDS ) of the high-pressure column ( 13 ) and is purified ( 3 ) at about this first pressure (p 2 ) . The purified total airstream ( 4 ) is divided into the first ( 5 ) and the second (6) feed airstream. The first feed airstream ( 5 ) is further compressed ( 8 ) separately from the second feed airstream to a second pressure (P 2 ) which is at least equal to the operating pressure (P HDS ) of the high-pressure column ( 13 ).

[0001] The invention relates to a process for producing a pressurizedproduct by low-temperature fractionation of air with production of agaseous pressurized product from a mixing column. In the invention, themixing column is operated at a pressure which is lower than theoperating pressure of the high-pressure column of the two-column systemwhich serves for nitrogen-oxygen separation. separation.

[0002] The rectification system of the invention can be constructed as atwo-column system, for example as a classic double-column system, orelse as a three-column or multiple column system. It can, additionallyto the columns for nitrogen-oxygen separation, have other apparatusesfor producing other air components, in particular noble gases (forexample krypton, xenon and/or argon).

[0003] The oxygen-rich fraction which is used as feed for the mixingcolumn has an oxygen concentration which is higher than that of air andis, for example, 70 to 99.5 mol%, preferably 90 to 98 mol%. Mixingcolumn is taken to mean a countercurrent contact column in which a morevolatile gaseous fraction is charged in countercurrent to aless-volatile liquid.

[0004] The inventive process is suitable, in particular, for producinggaseous impure oxygen under pressure. Impure oxygen is here termed amixture having an oxygen content of 99.5 mol% or less, in particular 70to 99.5 mol%. The product pressures are, for example, 2.2 to 4.9 bar,preferably 2.5 to 4.5 bar. Obviously, the pressurized product, asrequired, can be further compressed in the gaseous state.

[0005] A process and an apparatus of the type mentioned at the outsetare disclosed by EP 697576 Al Here, the total air is compressed to abouthigh-pressure column pressure and the mixing column air is then expandedto the operating pressure of the mixing column, with a part of themixing column air being work-expanded. By this means, although the highpressure of this partial airstream can be used to produce cold, theknown process is not energetically favourable in all cases.

[0006] The object underlying the invention is to specify a process ofthe type mentioned at the outset and a corresponding apparatus whichhave a particularly low energy consumption.

[0007] This object is achieved by means of the fact that a totalairstream which comprises at least the first and second feed airstreamsis compressed to a first pressure (p₁) which is lower than the operatingpressure (p_(HDS)) of the high-pressure column and is advantageouslypurified at about this first pressure (p₁) that the purified totalairstream is divided into the first and second feed airstreams and thatthe first feed airstream is further compressed separately from thesecond feed airstream to a second pressure (p₂) which is at least equalto the operating pressure (p_(HDS)) of the high-pressure column.

[0008] The total airstream is therefore not compressed to the highestpressure in the system, but to a lower value. The air fraction or airfractions which require a relatively high pressure, in particular thehigh-pressure-column air, are specifically separately furthercompressed. As a result, the process can proceed with the lowestpossible energy consumption in compression of the feed air.

[0009] The lowest equipment costs are achieved if the first pressure isabout equal to the operating pressure of the mixing column. In this casethe mixing column air (second feed airstream) can be introduced into themixing column without further pressure-changing measures.

[0010] Alternatively thereto, the first pressure can be lower than theoperating pressure (p_(M1S)) of the mixing column. In this case, thesecond feed airstream is further compressed separately from the firstfeed airstream to a third pressure (p₃) which is at least equal to theoperating pressure (p_(M1S)) of the mixing column.

[0011] Preferably, the oxygen-rich fraction brought to pressure in theliquid state, before being introduced into the mixing column, is warmedin indirect heat exchange with a superheated airstream. The superheatedairstream is formed, for example, by a portion of the feed air which isat high-pressure-column pressure. This is taken off at an intermediatetemperature from the main heat exchanger in which feed air is cooled toabout dewpoint, and brought to the indirect heat exchange with theoxygen-rich liquid without further temperature-changing measures. Inthis manner, the temperature of the liquid which is applied to themixing column is optimally matched to the conditions in thecountercurrent flow mass transfer within the mixing column.

[0012] Cold is produced in a favourable manner in the process by a thirdfeed airstream being work-expanded and introduced into the low-pressurecolumn. By this means the “natural” pressure drop between the firstpressure or another process pressure can be utilized to compensate forinsulation losses and, if appropriate, to liquefy a portion of theproducts.

[0013] Preferably, the third feed airstream, before the work expansion,is recompressed, in which case, in particular, mechanical energyproduced during the work expansion is used to drive the recompression.In this case a turbine-booster combination can be used in which theexpansion turbine and recompressor are mechanically coupled via a sharedshaft.

[0014] The third feed airstream can be compressed to the first pressureand purified together with the first and second feed airstreams. Then itis either immediately fed to the recompression or recompressed stilltogether with the first feed airstream.

[0015] Alternatively to injecting the third feed airstream into thelow-pressure column, the second feed airstream, after its furthercompression, and before being fed into the mixing column, car bework-expanded. The further compression is then carried out to a secondpressure which is markedly higher than the mixing column pressure.

[0016] The invention also relates to an apparatus for conducting theprocess.

[0017] The invention and further details of the invention are describedin more detail below with reference to exemplary embodiments shown inthe drawings. In this case:

[0018]FIG. 1 shows a process and an apparatus having work expansion of aportion of the air compressed to the first pressure,

[0019]FIG. 2 shows a modified process having work expansion of a portionof :he air compressed to the second pressure,

[0020]FIG. 3 shows a process having work expansion of the mixed-columnair and

[0021]FIG. 4 shows another variant of FIG. 1 without recompression ofthe turbine air.

[0022] In the case of the process shown in FIG. 1, feed air 1 is broughtin a two-stage air compressor 2 with after-cooling to a first pressurep₁ of, for example, 2.7 to 3.7 bar, preferably about 3.2 bar and, atpreferably this pressure, enters into a purification device 3, which ispreferably formed by a pair of conventional molecular-sieve adsorbers.The purified total air 4 divided into three partial streams 5, 6, 7.

[0023] The first feed airstream 5 is further compressed in a firstrecompressor 8 to a second pressure p₂ of, for example, 4.4 to 7.0 bar,preferably about 5.7 bar, and, after after-cooling 9 flows into a mainheat exchanger 10. The first feed airstream leaves the main heatexchanger 10 via line 11 at about dewpoint temperature and is fed vialine 12 into a high-pressure column 13. The operating pressure P,.DS ofthe high-pressure column 13 is, for example, 4.3 to 6.9 bar, preferablyabout 5.6 bar. The rectification system in addition has a low-pressurecolumn 14 which is operated at, for example, 1.3 to 1.7 bar, preferablyabout 1.5 bar.

[0024] The second feed airstream C is also passed through the main heatexchanger 10 at about the first pressure p₁ (minus pipe losses andpressure drops in the cleaning device) and finally flows via line 15 tothe mixing column. The feed point is directly above the bottom of themixing column 16.

[0025] The third partial stream 7 is recompressed from about the firstpressure P, to a third pressure p, of, for example, 3.8 to 5.6 bar,preferably about 4.7 bar, in a second recompressor 17 and, afterafter-cooling 18 is fed via line 19 to the warm end of the main heatexchanger. However, it is only cooled to an intermediate temperature andis taken off again from the main heat exchanger 10 before the cold endvia line 50 and work-expanded in a turbine 20. The expanded air 21 isinjected into the low-pressure column 14. Recompressor 17 and turbine 20are directly mechanically coupled.

[0026] The rectification system is designed in the exemplary embodimentsas a classic Linde double-column apparatus having a condenser-evaporator22 as main condenser. However, the invention can also be used inrectification systems having other condenser and/or columnconfigurations.

[0027] Oxygen-enriched liquid 23 from the bottom of the high-pressurecolumn 13 is cooled in a first sub-cooling countercurrent heat exchanger24 and fed, after throttling 25, to the low-pressure column 14, at anintermediate point 26. Gaseous nitrogen 27 from the top of thehigh-pressure column 13 can in part 22 be warmed in the main heatexchanger 10 and produced as pressurized nitrogen product 29. Theremainder 30 Is essentially completely condensed in the main condenser22. The liquid nitrogen 31 produced here is at least in part 32introduced as reflux into the high-pressure column 13. If required,another portion 33 can be taken off as liquid product. An intermediateliquid (impure nitrogen) of the high-pressure column 34 serves, aftersub-cooling 24 and throttling 35, as reflux for the low-pressure column.Gaseous impure nitrogen 36 from the top of the low-pressure column iswarmed in the heat exchangers 24 and 10 and finally taken off via line37. It can be used, as shown, as regeneration gas for the purificationdevice 3.

[0028] Liquid oxygen 38 is taken off from the bottom of the low-pressurecolumn brought to a pressure of, for example, 5.7 to 6.5 bar, preferablyabout 6.1 bar, in a pump 39, warmed in a second sub-coolingcountercurrent heat exchanger 40 and finally introduced (41) into thetop of the mixing column 16. In the second sub-cooling countercurrentheat exchanger 40, in particular, a superheated airstream 42 is cooledwhich is branched off from the first feed airstream upstream of the coldend of the main heat exchanger, more precisely at an intermediatetemperature which is lower than the inlet temperature of the turbine 20.This airstream, after its cooling, is recombined via line 43 with thefirst feed airstream 11. Via the valve 44, the amount of the airstreamflowing through the second sub-cooling countercurrent heat exchanger isset.

[0029] From the top of the mixing column 16, gaseous impure pressurizedoxygen 51 is taken off, warmed in the main heat exchanger 10 andproduced as product 52. Bottoms liquid 45 and an intermediate liquid 46are taken off from the mixing column and fed via the lines 47 and 48,respectively, to the low-pressure column 14 at a suitable point.

[0030]FIG. 2 differs from Figure only in that the third feed airstream207 is further compressed together with the second feed airstream in thefirst recompressor 108. As a result, a higher inlet pressure is reachedat the turbine 20 and correspondingly more cold is produced.

[0031] In the variant of FIG. 3, the purification device is operated ata first pressure P, which is higher than the operating pressure p_(M1S)of the mixing column. The first pressure p₁' is here, or example, 2.7 to3.7 bar, preferably about 3.2 bar. Here the second feed airstream 306 isexpanded upstream of its feed into the mixing column. A third feedairstream which is injected into the low-pressure column does not exist.The second feed airstream 306 is further compressed downstream of itsbranching off from the purified total air in the second recompressor317, which is driven by the turbine 320. The second feed airstream 349which is further compressed, for example, to 3.8 to 5.6 bar, preferablyabout 4.7 bar, is fed via line 350 to the turbine 320 and iswork-expanded there to about mixed-column-pressure p_(M1S)

[0032] Similarly to the case in FIG. 3, in the process of FIG. 4, thepurification 3 is operated at a particularly low first pressure p₁′′ of,for example, 2.7 to 3.7 bar, preferably about 3.2 bar. The turbine 420is, as in FIG. 1, subjected to a third feed airstream 407, 450 whichhere, however, is not recompressed, but is directly work-expanded fromabout the first pressure p₁to about low-pressure-column pressure. Therecompressor 418 driven by the turbine is here used for furthercompression of the second feed airstream to the second pressure p₂,which is about equal to the operating pressure p_(M1S) of the mixingcolumn.

[0033] In all exemplary embodiments, the air compressor and therecompressor 8, 108 are preferably jointly constructed as a three-stagemachine. In other words, the further compression of the first feedairstream is carried out in the third stage of a machine whose first andsecond stages serve for air compression upstream of the purification 3.Alternatively thereto, this machine can also be constructed in fourstages, with in tis case the first three stages being arranged beforethe purification device 3.

[0034] Referring back to the process, whereas it is advantageous for thecompressed total air stream to be purified at the compression pressure,it is also possible that purification can be conducted upstream ordownstream for individual feed streams and/or at different pressures.

[0035] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples. Also, the preceding specific embodiments are to be construedas merely illustrative, and not limitative of the remainder of thedisclosure in any way whatsoever.

[0036] The entire disclosure of all applications, patents andpublications, cited above and below, and of corresponding Germanapplication 10015602.9, are hereby incorporated by reference.

[0037] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A process for producing a pressurized product by low-temperaturefractionation of air in a rectification system which has a high-pressurecolumn (13) and a low-pressure column (14), in which a first feedairstream (12) is introduced into the high-pressure column (13), anoxygen-rich fraction (38) from the low-pressure column (14) is brought(39) to pressure in the liquid state and introduced (41) into a mixingcolumn (16), a second feed airstream (6, 15, 306, 406) is introducedinto the lower region of the mixing column (16) and brought intocountercurrent contact with the oxygen-rich fraction (41), with themixing column (16) being operated at a pressure (p_(MIS)) which is lowerthan the operating pressure (p_(HDS)) of the high-pressure column (13),and in which a gaseous top product (51) is taken off from the upperregion of the mixing column (16) and produced as pressurized product(52), characterized in that a total airstream (1) which comprises thefirst and second feed airstreams is compressed (2) to a first pressure(p₁) which is lower than the operating pressure (p_(HDS)) of thehigh-pressure column (13), the total airstream (4) is divided into thefirst (5) and the second (6, 306, 406) feed airstreams and in that thefirst feed airstream (5) is further compressed (8, 108) separately fromthe second feed airstream to a second pressure (p₂) which is at leastequal to the operating pressure (p_(HDS)) of the high-pressure column(13).
 2. Process according to claim 1 , characterized in that the firstpressure (p₁) is about equal to the operating pressure (p_(M1S)) of themixing column (16).
 3. Process according to claim 1 , characterized inthat the first pressure is lower than the operating pressure (p_(M1S))of the mixing column (16) and in that the second feed airstream (306,406) is further compressed (317, 417) separately from the first feedairstream to a third pressure (p₃) which is at least equal to theoperating pressure (p_(M1S)) of the mixing column (16).
 4. Processaccording to one of claims 1 to 3 , characterized in that theoxygen-rich fraction brought to pressure in the liquid state, before theintroduction (41) into the mixing column (16), is warmed in indirectheat exchange (40) with a superheated airstream (42).
 5. Processaccording to one of claims 1 to 4 , characterized in that a third feedairstream (7, 50, 207, 407, 450) is work-expanded (20, 420) andintroduced (21) into the low-pressure column (14).
 6. Process accordingto one of claims l to 5, characterized in that the third feed airstream(7), before the work expansion (20) , is recompressed (17) with, inparticular, mechanical energy produced during the work expansion (20)being used for the recompression (17).
 7. Process according to one ofclaims 1 to 6 , characterized in that the third feed airstream is formeddownstream of the purification (3) by a portion of the total airstream(4) and is fed (7, 207) directly, or after joint recompression (108)with the first feed airstream, to the recompression (17).
 8. Processaccording to one of claims 1 to 4 , characterized in that the secondfeed airsteam (306, 349, 350) is work-expanded (320) before itsintroduction into the mixing column.
 9. Apparatus for producing apressurized product by low-temperature fractionation of air by arectification system which has a high-pressure column (13) and alow-pressure column (14) and having a first feed air line (5, 11, 12)which leads into the high-pressure column (13), a liquid line (38, 41)for taking off an oxygen-rich fraction from the low-pressure column(14), which comprises means (39) for pressure elevation and leads to amixing column (16), a second feed air line (6, 15) which leads to thelower region of the mixing column (16), and having an oxygen productline which is connected to the upper region of the mixing column (16),characterized by a total air line which leads via an air compressor (2)and a purification device (3) and is connected downstream to the firstand second feed air lines and by a recompressor (8, 108) which isdisposed in the first feed air line (5, 11, 12).